1. Field of the Invention
The invention relates to plural image display, such as picture-in-picture (PIP) or double-window displays.
2. Description of the Related Art
U.S. Pat. No. 5,369,442 discloses a method for picture-in-picture insertion, and a device for performing the method, in which successive frames of a small picture to be inserted into a main picture having successive frames are alternatingly written frame-wise into a memory region of a memory device. A decision signal is generated at a beginning of a display of the main picture, for deciding from which of the two memory regions a stored frame of the small picture is to be read out. Frames of the small picture are read out from whichever memory region enables joint-line-free insertion of the small picture into the main picture.
In PIP or double-window applications, one live video signal is inserted in another live video signal. Generally, the two video signals are independent of each other and therefore can differ in frequency (field repetition rate). This means that one of the signals has to be synchronized to the other (on field base). The minimum amount of memory for the synchronizing circuit is one field. However, if only one field can be stored in memory, an artifact in the resulting video signal will occur, which is referred to as xe2x80x98joint line errorxe2x80x99. A joint line error occurs if an outgoing field is built up of two subsequent input fields and is caused the read and write pointers of the field memory crossing each other. The standard approach in PIP applications to prevent a joint line error is to store two subsequent fields in memory and choosing to read the field which is not currently been written. In this way, the write pointer never crosses the read pointer and no joint line error will occur. The drawback is that it requires twice the amount of memory.
It is, inter alia, an object of the invention to use less memory in coping with the joint line error problem. To this end, a first aspect of the invention provides a memory control method and device. A second aspect of the invention provides a display apparatus for displaying at least two images.
In a method of controlling a memory to allow for a display of at least two images in accordance with a primary aspect of the present invention, write and read speeds of writing image data into and reading image data from the memory are measured to predict a crossing where a write action overtakes a read action, or reversely, where a new field of said image data is written into the memory from a same initial position as from which a previous field of the image data was written into the memory if no crossing is predicted, and the new field of said image data is written from an end position in the memory at which an end of the previous field of the image data was written into the memory if a crossing is indeed predicted, the memory having a size being larger than that needed for one field but less than that needed for two fields of the image data at its largest read-out size. If the method is only used for PIP, the read-out size is the size of a PIP image, while if the method is used for (both PIP and) double-window display, the largest read-out size is that of the left-hand or right-hand half of the double-window display.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.